1. Field of the Invention
The present invention relates to RF repeaters for use in cordless telephone systems and, more particularly, for interfacing between cordless handsets and base stations by exchanging downstream transmit signals from a base station and upstream receive signals from cordless handsets using time division duplex transmissions.
The present RF repeater is useful in particular in telephone systems employing cable television plant as a signal conduit but may also be employed in cordless telephone systems utilizing dedicated coaxial cable and/or fiber optic and/or microwave signal conduits.
2. Description of the Related Art
It is expected that Personal Communication Services (PCS) microcells will be supporting a rapidly increasing number of handsets in North America in the near future. To support this user base it is essential that the PCS microcells be both low power (to assist frequency re-use) and low cost (because the net capital costs of the PCS microcells will be a major factor in the economic viability of PCS).
What has been suggested by a number of organisations is that existing cable television distribution plant be used to interconnect microcell equipment. Taking advantage of the broadband and the nearly ubiquitous nature of cable plant, it has been further proposed that the microcell equipment consist of simple RF repeaters that translate off-air mobile voice traffic onto the cable plant and vice versa.
This approach uses the cable plant as a RF combining/splitting network since it preserves the basic RF amplitude and phase/frequency information. What has become apparent in tests is that this approach to PCS microcells yields both low capital costs and improved user service.
In summary, the low cost arises from the combination of simple technology (an RF repeater), using an existing asset base (i.e. cable plant) in a fashion that allows modulation/demodulation and PSTN interface equipment to be centrally located. This allows these equipment costs to be amortised over a very large net coverage area.
The improved service arises from better call blocking probability associated with the ability to centralise the base station equipment rather than a priori allocation to specific microcells. Additionally, the cable plant can act to form distributed antenna arrays that can be shaped into "roamer corridors". Within these roamer corridors it is also possible to control the off-air dynamic range so as to reduce near user/far user interactions and like of sight blocking.
FIG. 1 illustrates the principal hardware elements and concepts of a prior art cordless telephone system employing base stations.
Base stations 1 operate at the off-air frequencies and perform demodulation and modulation functions for the telephone signals. The base stations 1 interface directly to twisted pair telecom lines.
The base stations 1 can be mounted to interface directly with nearby handsets (not shown), or can be located at a central site, as shown, where their ability to handle calls can be amortised over a larger network of microcells connected by TV cable plant, as mentioned above.
Remote antenna signal processors (RASPs) 2 are located at the central site and interface the base stations to cable plant 4.
Typically, signals from the base stations 1 travel over the cable plant to the handset in the 200-450 MHz band. Signals travelling in the reverse direction use the 5-30 MHz return band on the cable plant.
Bi-directional distribution amplifiers 6 need to be compatible with the cable plant 4 and provide return band capability.
Remote antenna drivers (RADs) 8 must be compatible with existing TV cable plant and they may be configured for either coax or fiber plant.
RADs 8 pick-up the off-air signal and relay it back to a central site via the plant's return path, and also broadcast PCS signals on the cable downstream path (200-450 MHz) to nearby handsets, after suitable heterodyne operations.
This prior art RAD - RASP design suffers a number of limitations, which comprise, specifically:
The need to operate where there is cable television plant. Cable TV is readily available for residential markets, but less available or not at all available in public and business markets.
The need for compatibility with existing cable TV services. This requires the RAD - RASP units to use expensive heterodyne processing to interface time division duplex off-air signals to the frequency division duplex cable TV plant.
The RAD - RASP arrangement is inappropriate in some markets, e.g. those served by existing cordless base stations and those with predominately buried cable plant.
Furthermore, with such a system, the voice quality within a distributed antenna can suffer some degradation from the differences in phase noise of its constituent parts and from the differences in time delay of its constituent parts.
In some circumstances, therefore, it is preferable to use RF repeaters, transferring signals exclusively by a time division duplex protocol, instead of RADs, which employ frequency division duplexing.
Prior art RF repeaters have required two amplifiers, for amplifying the downstream transmit signals and the upstream receive signals, respectively, in each RF repeater.